The present invention relates to an optical fiber strain and temperature measurement apparatus and an optical fiber strain and temperature measurement method using Brillouin scattered light.
With the evolution of optical fiber communications, distributed optical fiber sensing, in which the optical fiber itself serves as the sensing medium, has become an active area of research. Representative distributed optical fiber sensing is optical time domain reflectometry (OTDR), in which optical pulses are launched into an optical fiber from one end of the optical fiber, and light backscattered within the optical fiber is measured with respect to time. Backscattering in an optical fiber includes Rayleigh scattering, Brillouin scattering, and Raman scattering. Among others, OTDR that measures spontaneous Brillouin scattering is referred to as Brillouin OTDR (BOTDR) (see, for example, T. Kurashima et al., “Brillouin Optical-fiber time domain reflectometry”, IEICE Trans. Commun., vol. E76-B, no. 4, pp. 382-390 (1993)).
Brillouin scattering can be observed at frequencies frequency-shifted on Stokes and anti-Stokes side with the frequency shift of the order of GHz with respect to the center frequency of the optical pulse launched into the optical fiber. The spectrum of Brillouin scattering is referred to as the Brillouin gain spectrum (BGS). The frequency shift and the spectral line width of the BGS are referred to as the Brillouin frequency shift (BFS) and the Brillouin line width, respectively. The BFS and the Brillouin line width vary depending on the material of the optical fiber and the wavelength of the incident light. For example, in the case of silica-based standard single-mode optical fiber, it is reported that, for an incident wavelength of 1.55 μm, the frequency shift amounts of the BFS and the Brillouin line width are approximately 11 GHz and approximately 30 MHz, respectively. Also, according to T. Kurashima et al., “Brillouin Optical-fiber time domain reflectometry”, IEICE Trans. Commun., vol. E76-B, no. 4, pp. 382-390 (1993), the frequency shift amounts of the BFS associated with strain and a temperature change inside a single-mode fiber are 0.049 MHz/με and 1.0 MHz/° C., respectively, for an incident wavelength of 1.55 μm.
Here, since the BFS has dependencies on strain and temperature, BOTDR has been attracting attention for the purpose of monitoring large constructions represented by bridges and tunnels, potential areas of landslide occurrence, or the like.
BOTDR generally performs heterodyne detection to measure spectrum waveform of spontaneous Brillouin scattered light arising in an optical fiber with the use of reference light prepared separately. The intensity of spontaneous Brillouin scattered light is lower than the intensity of Rayleigh scattering light by two through three orders of magnitude. Thus, heterodyne detection is useful in increasing the minimum light receiving sensitivity.
With reference to FIG. 11, conventional BOTDR (see, for example, JP 2001-165808A) is described below. FIG. 11 is a schematic block diagram illustrating the conventional optical fiber strain measurement apparatus.
Continuous light launched from a light source 112 is split into two branches by an optical coupler 142. One of the two branches is used as reference light, and the other is frequency-shifted by a frequency shift amount that corresponds to the Brillouin frequency by an optical frequency shifter 143 and then is converted into pulsed probe light by an optical pulse generator 114.
The probe light is launched into an optical fiber to be measured (optical fiber under test) 100 via an optical coupler 120. Brillouin backscattered light from the optical fiber under test 100 is multiplexed with the reference light at an optical coupler 150 and then is heterodyne detected by a receiver 160 which is composed of a balanced photodiode (PD) 162 and an FET amplifier 164.
Here, since the probe light is frequency-shifted as much as the Brillouin frequency shift by the optical frequency shifter 143, the frequency of the beat signal generated through the heterodyne detection is low. This allows the PD 162 and the PET amplifier 164 for low frequency bandwidth to be used at the receiving side. The beat signal is frequency-shifted down with a mixer 170 and an electrical filter 178, and then is square detected or envelope detected with a detection circuit 172, thereby providing intermediate frequency (IF) signal. The power or amplitude of the IF signal is measured to transmit the measured result to a signal processing device 174.
Note that, since BOTDR deals with information on frequency spectrum distribution along the length of the optical fiber, it is necessary to obtain three-dimensional information with axes of time, amplitude and frequency. With reference to FIG. 12, a method for obtaining the three-dimensional information with axes of time, amplitude and frequency in BOTDR is described below. FIG. 12 is a schematic diagram for illustrating a method for obtaining the three-dimensional information with axes of time, amplitude and frequency in a conventional optical fiber strain measurement apparatus. In the technique disclosed in JP 2001-165808A described above, two-dimensional information with axes of time t and amplitude I is obtained by sweeping frequency f of a local electrical signal source 183 in order to measure the entire Brillouin frequency spectrum.
Here, not limited to BOTDR, in distributed optical fiber sensing using Brillouin scattering, both strain and a temperature change affect the BFS, as described above. Accordingly, it is essential to discriminate between strain and temperature effects. To achieve this object, a method is proposed that utilizes a coefficient of strain dependence and a coefficient of temperature dependence of a Brillouin backscatter coefficient in an optical fiber (see, for example, T. R. Parker et al., “Simultaneous distributed measurement of strain and temperature from noise-initiated Brillouin scattering in optical fibers”, IEEE J. Quantum Electron., vol. 34, No. 4, pp. 645-659 (1998) or Y. Sakairi et al., “A system for measuring temperature and strain separately by BOTDR and OTDR”, Proceeding of SPIE, vol. 4920, pp. 274-284 (2002)).
It is reported that the frequency shift and the scattering coefficient both have temperature and strain dependency in Brillouin backscatter. When the coefficient of strain dependence and the coefficient of temperature dependence of the BFS are Cνε and CνT, respectively, and the coefficient of strain dependence and the coefficient of temperature dependence of the Brillouin scattering coefficient are CPε and CPT, respectively, strain and temperature effects can be separated by preliminarily measuring these coefficients and solving the following simultaneous equations (a) with two unknowns.
                                          δ            ⁢                                                  ⁢                          v              B                                =                                                    C                                  v                  ⁢                                                                          ⁢                  ɛ                                            ⁢              δ              ⁢                                                          ⁢              ɛ                        +                                          C                vT                            ⁢              δ              ⁢                                                          ⁢              T                                      ⁢                                  ⁢                              100            ⁢                                          δ                ⁢                                                                  ⁢                                  P                  B                                                            P                B                                              =                                                    C                                  P                  ⁢                                                                          ⁢                  ɛ                                            ⁢              δ              ⁢                                                          ⁢              ɛ                        +                                          C                PT                            ⁢              δ              ⁢                                                          ⁢              T                                                          (        a        )            
In the equations, δνB is a frequency shift amount of the BFS, δPB/PB is a relative amount of change of Brillouin scattering intensity. These δνB and δPB/PB are values measured in BOTDR. Also, δε and δT are amounts of strain and a temperature change, respectively.